Implantable systemic blood pressure measurement systems and methods

ABSTRACT

Implantable systems, and methods for use therewith, for monitoring arterial blood pressure on a chronic basis are provided herein. A first signal indicative of electrical activity of a patient&#39;s heart, and a second signal indicative of mechanical activity of the patient&#39;s heart, are obtained using implanted electrodes and an implanted sensor. By measuring the times between various features of the first signal relative to features of the second signal, values indicative of systolic pressure and diastolic pressure can be determined. In specific embodiments, such features are used to determine a peak pulse arrival time (PPAT), which is used to determine the value indicative of systolic pressure. Additionally, a peak-to-peak amplitude at the maximum peak of the second signal, and the value indicative of systolic pressure, can be used to determine the value indicative of diastolic pressure.

PRIORITY CLAIM

This application is a Divisional application of and claims priority andother benefits from U.S. patent application Ser. No. 11/848,586(Attorney Docket No. A07P3032), filed Aug. 31, 2007, entitled“IMPLANTABLE SYSTEMIC BLOOD PRESSURE MEASUREMENT SYSTEMS AND METHODS”,incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

Embodiments of the present invention relate to implantable systems formonitoring arterial blood pressure, and methods for use therewith.

BACKGROUND OF THE INVENTION

A person's circulatory system includes both systemic and pulmonarycirculation systems. Pulmonary circulation supplies the lungs with bloodflow, while the systemic circulation takes care of all the other partsof the body, i.e. the systemic circulation. The heart serves as a pumpthat keeps up the circulation of the blood. Both the pulmonary andsystemic circulatory systems are made up of arteries, arterioles,capillaries, venules and veins. The arteries take the blood from theheart, while the veins return the blood to the heart

Blood pressure is defined as the force exerted by the blood against anyunit area of the vessel wall. The measurement unit of blood pressure ismillimeters of mercury (mmHg). Pulmonary and systemic arterial pressuresare pulsatile, having systolic and diastolic pressure values. Thehighest recorded pressure reading is called systolic pressure, whichresults from the active contraction of the ventricle. Although thearterial pressure and indeed flow in the arteries is pulsatile, thetotal volume of blood in the circulation remains constant. The lowestpressure reading is called diastolic pressure which is maintained by theresistance created by the smaller blood vessels still on the arterialside of the circulatory system (arterioles). Stated another way, thesystolic pressure is defined as the peak pressure in the arteries, whichoccurs near the beginning of a cardiac cycle. In contrast, the diastolicpressure is the lowest pressure, which occurs at the resting phase ofthe cardiac cycle. The pulse pressure reflects the difference betweenthe maximum and minimum pressures measured (i.e., the difference betweenthe systolic pressure and diastolic pressure). The mean arterialpressure is the average pressure throughout the cardiac cycle.

Arterial pulse pressure, such as mean arterial pressure (MAP), is afundamental clinical parameter used in the assessment of hemodynamicstatus of a patient. Mean arterial pressure can be estimated from realpressure data in a variety of ways. Among the techniques that have beenproposed, two are presented below. In these formulas, SP is the systolicblood pressure, and DP is diastolic pressure.

${a.\mspace{14mu} {MAP}_{2}} = {{\left( {{SP} + {2\; {DP}}} \right)/3} = {{\frac{1}{3}({SP})} + {\frac{2}{3}({DP})}}}$b.  MAP₁ = (SP + DP)/2

Systolic pressure and diastolic pressure can be obtained in a number ofways. A common approach is to use a stethoscope, an occlusive cuff, anda pressure manometer. However, such an approach is slow, requires theintervention of a skilled clinician and does not provide timely readingsas it is a measurement at only a single point in time. While systolicpressure and diastolic pressure can also be obtained in more automatedfashions, it is not always practical to obtain measures of pressureusing a cuff and pressure transducer combination, especially if theintention or desire is to implant a sensor that can monitor arterialpressure on a chronic basis.

Another approach for obtaining measures of arterial pressure is to usean intravascular pressure transducer. However, an intravascular devicemay cause problems, such as, embolization, nerve damage, infection,bleeding and/or vessel wall damage. Additionally, the implantation of anintravascular lead requires a highly skilled physician such as asurgeon, electrophysiologist, or interventional cardiologist.

Plethysmography, the measurement of volume of an organ or body part, hasa history that extends over 100 years. Photoplethysmography (PPG) usesoptical techniques to perform volume measurements, and was firstdescribed in the 1930s. While best known for their role in pulseoximetry, PPG sensors have also been used to indirectly measure bloodpressure. For example, non-invasive PPG sensors have been used incombination with in an inflatable cuff in a device known as Finapres.U.S. Pat. Nos. 4,406,289 (Wesseling et al.) and 4,475,940 (Hyndman) areexemplary patents that relate to the Finapres technique. The cuff isapplied to a patient's finger, and the PPG sensor measures theabsorption at a wavelength specific for hemoglobin. After the cuff isused to measure the individual's mean arterial pressure, the cuffpressure around the finger is then varied to maintain the transmuralpressure at zero as determined by the PPG sensor. The Finapres devicetracks the intra-arterial pressure wave by adjusting the cuff pressureto maintain the optical absorption constant at all times.

There are a number of disadvantages to the Finapres technique. Forexample, when there exists peripheral vasoconstriction, poor vascularcirculation, or other factors, the blood pressure measured in a fingeris not necessarily representative of central blood pressure. Further,maintaining continuous cuff pressure causes restriction of thecirculation in the finger being used, which is uncomfortable whenmaintained for extended periods of time. Accordingly, the Finaprestechnique is not practical for chronic use. Additionally, because of theneed for a pneumatic cuff, a Finapres device can not be used as animplanted sensor.

Simple external blood pressure monitors also exist, but they do notoffer continuous measurement and data logging capability. These devicescan be purchased at a drug store, but patient compliance is required tomake regular measurements and accurately record the data. Additionally,portable external miniature monitors that automatically log bloodpressure data exist, but these devices can only store a day or so ofdata and require clinician interaction to download and process themeasured data.

As is evident from the above description, there is the need for improvedsystems and methods for monitoring arterial blood pressure, includingsystolic pressure, diastolic pressure and mean arterial pressure.

SUMMARY OF THE INVENTION

Embodiments of the present invention relate to implantable systems, andmethods for use therewith, for monitoring a patient's arterial bloodpressure. Implanted electrodes are used to obtain a first signalindicative of electrical activity of the patient's heart, such as anintracardiac electrogram (IEGM) as recorded from electrodes placedwithin the heart or electrocardiogram (ECG) as recorded from electrodesin a subcutaneous location. Additionally, an implanted sensor is used toobtain a second signal indicative of mechanical activity of thepatient's heart. For example, an implanted plethysmography sensor can beused to obtain a plethysmography signal. In specific embodiments, theimplanted plethysmography sensor is a photoplethysmography (PPG) sensorthat obtains a PPG signal.

In accordance with specific embodiments, a ventricular depolarizationand a ventricular repolarization is detected in a portion of the firstsignal corresponding to a cardiac cycle. Ventricular depolarization canbe detected by detecting an R-wave in the portion of the first signalcorresponding to the cardiac cycle, and ventricular repolarization canbe detected by detecting a T-wave in the portion of the first signalcorresponding to the cardiac cycle. Additionally, a maximum peakamplitude is detected in a portion of the second signal corresponding tothe same cardiac cycle. This enables specific times to be determined,including a time t₁ from the detected ventricular depolarization to thedetected maximum peak amplitude in the second signal, and a time t₂ fromthe detected ventricular repolarization to the detected maximum peakamplitude in the second signal.

Based on the times t₁ and t₂, a peak pulse arrival time (PPAT) isdetermined. For example, the PPAT can be the mean of times t₁ and t₂,but use of alternative equations are also possible. A value indicativeof systolic pressure (SP) can then be determined based on the PPAT.

In alternative embodiments, t₁, but not t₂, is determined, and a pulsearrival time (PAT) is determined based on t₁. In such embodiments, avalue indicative of SP is determined based on the PAT.

Additionally, a peak-to-peak amplitude a₁ in the second signal can bedetermined, and a value indicative of diastolic pressure (DP) can bedetermined based on the amplitude a1 and the value indicative of SP.This can include determining a value indicative of pulse pressure (PP)based on the amplitude a₁, and determining a value indicative ofdiastolic pressure (DP) by subtracting the value indicative of PP fromthe value indicative of SP. Values of SP and DP can be stored in memoryof the implantable system, and data indicative of the stored valuesindicative of SP and DP can be wireless transmitted to a non-implanteddevice, e.g., for display to a physician.

In accordance with specific embodiments of the present invention, theabove described method is repeated over time to thereby track changes inSP and DP. In specific embodiments, an activity sensor and/or posturesensor can be used to trigger the performance of the above describedmethod.

In certain embodiments, an alarm can be triggered based on comparisonsof the values indicative of SP, the values indicative of DP, the changesin SP and/or the changes in DP, to corresponding thresholds. Such analarm can be part of the implanted system, or a non-implanted alarm canbe triggered.

Other features and advantages of the invention will appear from thefollowing description in which the preferred embodiments have been setforth in detail, in conjunction with the accompanying drawings andclaims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 includes signal waveforms that are used to show the relativetiming of electrical and mechanical cardiac events that occur during acardiac cycle. The upper graph includes an aortic pressure waveform, aleft atrial pressure waveform and a left ventricular pressure waveform.The middle graph includes a signal indicative of electrical cardiacactivity. The lower graph includes a photoplethysmography (PPG) signal,which is indicative of mechanical cardiac activity.

FIG. 2A is a high level flow diagram that is used to explain specificembodiments of the present invention.

FIG. 2B is a high level flow diagram that is used to explain alternativeembodiments of the present invention.

FIG. 3A illustrates an exemplary implantable stimulation device thatincludes a PPG sensor, and which can be used to perform embodiments ofthe present invention.

FIGS. 3B and 3C illustrates exemplary implantable monitoring devicesthat include a PPG sensor, and which can be used to perform embodimentsof the present invention.

FIG. 4 is a simplified block diagram that illustrates possiblecomponents of the implantable devices shown in FIGS. 3A-3C.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best modes presently contemplatedfor practicing various embodiments of the present invention. Thedescription is not to be taken in a limiting sense but is made merelyfor the purpose of describing the general principles of the invention.The scope of the invention should be ascertained with reference to theclaims. In the description of the invention that follows, like numeralsor reference designators will be used to refer to like parts or elementsthroughout. In addition, the first digit of a reference numberidentifies the drawing in which the reference number first appears.

It would be apparent to one of skill in the art that the presentinvention, as described below, may be implemented in many differentembodiments of hardware, software, firmware, and/or the entitiesillustrated in the figures. Any actual software, firmware and/orhardware described herein is not limiting of the present invention.Thus, the operation and behavior of the present invention will bedescribed with the understanding that modifications and variations ofthe embodiments are possible, given the level of detail presentedherein.

Referring to FIG. 1, the signal waveforms therein are used to show therelative timing of electrical and mechanical cardiac events that occurduring a cardiac cycle. The upper graph includes an aortic pressurewaveform 102, a left atrial pressure waveform 104 and a left ventricularpressure waveform 106. The middle graph includes an electro-cardiogram(ECG) or intracardiac electrogram (IEGM) waveform 108, which is shown asincluding a P wave, a ORS complex (including Q, R and S waves) and a Twave. The P wave is caused by depolarization of the atria. This isfollowed by atrial contraction, which is indicated by a slight rise inthe atrial pressure (seen in waveform 104) contributing to furtherfilling of the ventricle. Following atrial contraction is ventriculardepolarization, as indicated by the QRS complex, with ventriculardepolarization initiating contraction of the ventricles resulting in arise in ventricular pressure until it exceeds the pulmonary and aorticdiastolic pressures to result in forward flow as the blood is ejectedfrom the ventricles. Ventricular repolarization occurs thereafter, asindicated by the T wave and this is associated with the onset ofventricular relaxation in which forward flow stops, the pressure in theventricle falls below that in the atria at which time the mitral andtricuspid valves open to begin to passively fill the ventricle duringdiastole. Also shown in FIG. 1, in the bottom graph, is aphotoplethysmography (PPG) signal 110, which will be described inadditional detail below.

In accordance with specific embodiments of the present invention, anIEGM signal (e.g., like 108) is obtained using implanted electrodes onendocardial lead(s), which typically provide for better fidelity than anECG signal obtained from non-implanted surface electrodes. Additionally,a signal indicative of mechanical activity of a patients heart, such asa plethysmography signal (e.g., like 110), is obtained from an implantedsensor. In accordance with specific embodiments of the presentinvention, by detecting the timing and amplitude of certain features ofsuch signals, various arterial blood pressure measurements can beobtained, including systolic pressure, diastolic pressure, pulsepressure and/or mean arterial pressure. As mentioned above, the systolicpressure (SP) is the peak pressure in the arteries, which occurs nearthe beginning of a cardiac cycle. The diastolic pressure (DP) is thelowest pressure in the arteries, which occurs at the end of the restingphase of the cardiac cycle. The pulse pressure (PP) is the differencebetween the systolic and diastolic pressures. The mean arterial pressure(MAP) is a weighted average of pressure throughout the cardiac cycle.

Because implanted electrodes and an implanted sensor are used to obtainthe various arterial pressure measurements, a patient's arterial bloodpressure can be monitored on a chronic basis. Thus, arterial bloodpressure can be tracked to monitor a patient's worsening (or improving)cardiac disease state, and to trigger alerts and/or titration of bloodpressure medications. Additionally, arterial blood pressure measurementscan be used as a measure of hemodynamic function, and thus used in aclosed loop for hemodynamic optimization (e.g., A-V delay, VV delay,and/or pacing rate optimization).

Embodiments of the present invention can be implemented within apacemaker or ICD system, or as part of an implantable monitor that doesnot pace and/or shock a patient's heart. Additional details of suchembodiments are provided below.

Embodiments of the present invention use the concept of pulse arrivaltime, also known as pulse transmit time, or pulse wave velocity tomonitor arterial blood pressure. However, embodiments of the presentinvention differ from most prior art systems that rely on pulse arrivaltime, because most prior art systems are non-implanted. Accordingly,most prior are systems that rely on pulse arrival time are not practicalfor chronic use.

The inventors of the present invention are aware of one prior artreference, i.e., U.S. Pat. No. 4,425,920 (Bourland et al.), that doessuggest an implantable system for monitoring arterial blood pressureusing the concept of pulse transmit time. However, the system of the'920 patent requires that two sets of electrodes be positioned adjacentan artery at two sites, and thus, requires very precise and potentiallydifficult implantation of its system. In contrast, the implantablesystems of the present invention can be implanted in the same manner asany conventional pacemaker/ICD is implanted, or potentially in a simplermanner (if the system of the present invention is implemented in amonitor that does not pace and/or shock a patient's heart).

Additionally, many embodiments of the present invention use a novelmeasure, referred to below as peak pulse arrival time (PPAT), which isbelieved to provide for improved measures of arterial blood pressure.Referring to FIG. 1, it can be seen that a time of the peak arterialblood pressure, represented by dashed line 112, occurs at a time betweenventricular depolarization (as represented by the QRS complex) andventricular repolarization (as represented by the T wave). Morespecifically, it can be seen that the peak occurs generally halfwaybetween the QRS complex and the T wave. PPAT is determined, taking thisinto account.

As can also be seen from FIG. 1, the peak in the PPG signal 110 occursat a time after the peak in the arterial blood pressure (as shown in theupper graph). This is because the peak in the PPG signal 110 isindicative of the peak wave in arterial blood pressure generated by thepatient's heart, as detected by a PPG sensor located a distance from thepatient's heart. Presuming the PPG sensor is implanted in the pectoralregion of the patient (which is an option, but not necessary), the timethat it takes a peak pulse wave (as detected from ECG/IEGM electrodes)to travel from the patient's heart to the PPG sensor can be, e.g., onthe order of 10-100 msec, depending on the location of the electrodes(used to obtain the ECG/IEGM) and the location of the PPG sensor. Thepeak pulse wave is initially detectable from an ECG/IEGM obtained usingimplanted electrodes. The time at which the peak wave reaches theimplanted PPG sensor is detectable from a PPG signal produced by theimplanted PPG sensor. Accordingly, in accordance with embodiments of thepresent invention, the amount of time it takes a peak pulse wave totravel from the patient's heart to the PPG sensor can be determined.Such information is used to determine values of arterial blood pressure.It is also possible, and within the scope of the present invention, thatthe time it takes a peak pulse to travel from the patient's heart to thePPG sensor can be outside the 10-100 msec range mentioned above.

Embodiments of the present invention will first be summarized withreference to the high level flow diagrams of FIGS. 2A and 2B. Followingthe discussion of the flow diagrams, exemplary implantable systems ofthe present invention will be described, including discussions ofexemplary implantable electrodes and sensors that can be used. In theflow diagrams, the various algorithmic steps are summarized inindividual ‘blocks’. Such blocks describe specific actions or decisionsthat are made or carried out as the algorithm proceeds. Where amicrocontroller (or equivalent) is employed, the flow diagram presentedherein provides the basis for a ‘control program’ that may be used bysuch a microcontroller (or equivalent) to effectuate the desired controlof the implantable system. Those skilled in the art may readily writesuch a control program based on the flow diagram and other descriptionspresented herein.

Referring to FIG. 2A, at step 202, implanted electrodes are used toobtain a first signal indicative of electrical activity of the patient'sheart. The first signal can be an intracardiac electrogram (IEGM)obtained using one or more electrode of an endocardial lead, examples ofwhich are discussed below with reference to FIGS. 3A and 4.Alternatively, the first signal can be an electrocardiogram (ECG)obtained using one or more subcutaneous electrode. A portion of anexemplary first signal indicative of electrical activity of thepatient's heart is shown at 108 in FIG. 1.

At step 204, an implanted sensor is used to obtain a second signalindicative of mechanical activity of the patient's heart. In specificembodiments, the second signal can is a plethysmography signal, such as,but not limited to, a photoplethysmography (PPG) signal. An exemplaryPPG sensor, also referred to as an implanted optical sensor, isdiscussed below with reference to FIGS. 3A-3C and 4. An exemplaryportion of a PPG signal is shown at 110 in FIG. 1. Alternatively, thesecond signal can be an impedance plethysmography signal. In still otherembodiments, the second signal can be a signal output by a sensorincluding a piezo-electric diaphragm. Alternative sensors that can beused to produce the second signal, include, but are not limited to, aclose range microphone, a sensor including a small mass on the end of apiezo bending beam with the mass located on the surface of a smallartery, a transmission mode infrared motion sensor sensing across thesurface of a small artery, or a MEMS accelerometer located on thesurface of a small artery. Such alternative sensors can be located,e.g., on the tip of a short lead connected to a device header that issubcutaneously implanted in closed proximity to an implanted stimulationdevice. The implanted sensor is preferably extravascular, and preferablya sufficient distance from the patient's heart such that meaningfulchanges in the amount of time it takes a pulse wave originating in theheart to reach the implanted sensor can be detected, thereby enablingchanges in arterial blood pressure to be detected. For example, it ispreferred that the implanted sensor (used to obtain the second signalindicative of mechanical activity of the patient's heart) is at least 10mm from the patient's aortic root. Such a second sensor can beimplanted, e.g., in the pectoral region of a patient. Thus, it ispractical that the second sensor can be integrated with or attached tothe housing of a pacemaker or ICD, as can be appreciated from FIGS.3A-3C and 4 discussed below. Alternative locations for implantation ofthe second sensor include, but are not limited to, the patients leg, armor neck.

At step 206, a ventricular depolarization and a ventricularrepolarization are detected in a portion of the first signalcorresponding to a cardiac cycle. A QRS complex, such as the one shownin signal 108 of FIG. 1, is indicative of ventricular depolarization.Ventricular depolarization can be detected, e.g., by detected the Q waveof the QRS complex, the R wave of the QRS complex, and/or the S wave ofthe QRS complex. However, since the R wave is the easiest to detect, dueto its relatively large magnitude, it is practical for ventriculardepolarization to be detected by detecting the R wave. Accordingly, anyknown or future developed technique for detecting an R wave (e.g., bypeak detection or threshold crossing) can be used to detect ventriculardepolarization. Exemplary techniques for detecting R waves are disclosedin U.S. patent application Ser. No. 10/998,026, entitled “Systems andMethods for Detection of VT and VF from Remote Sensing Electrodes”(Nabutovsky et al.), filed Nov. 24, 2004 (Attorney Docket No. A04P1092),which is incorporated herein by reference. Alternatively, known orfuture developed techniques for detecting the Q, R and/or S waves can beused to detect ventricular depolarization.

A T wave, such as the one shown in signal 108 in FIG. 1, is indicativeof ventricular repolarization. Accordingly, any known or futuredeveloped technique for detecting a T wave can be used to detectventricular repolarization. Some exemplary techniques for detecting Twaves are disclosed in U.S. patent application Ser. No. 10/979,833,entitled “Systems and Methods for Automatically Setting Refractory andBlanking Periods,” (Snell and Bharmi) filed Nov. 1, 2004 (AttorneyDocket No. A04P1087), which is incorporated herein by reference. “Someadditional exemplary techniques for detecting T waves are disclosed inU.S. Pat. No. 5,782,887 (van Krieken et al) and U.S. Pat. No. 6,836,682(Van Dam), which are incorporated herein by reference. Use ofalternative techniques for detecting T waves are within are also withinthe scope of the present invention.”

At step 208, a maximum peak amplitude is detected in a portion of thesecond signal corresponding to the same cardiac cycle referred to instep 206. For the following discussion, it will be assumed that thesecond signal is a PPG signal. A peak detection circuit, a peakdetection algorithm or the like, can be used to detect the peakamplitude of a PPG signal (or other second signal), as is well known inthe art. As will be discussed below with reference to step 218, thepeak-to-peak amplitude a₁ at this point in the second signal (i.e., atthe point where the PPG signal amplitude is maximum) should also bedetermined. Thus, it would be practical to perform steps 208 and 218 atgenerally the same time.

At step 210, there is a determination of a time t₁ from detection of theventricular depolarization to the detection of the maximum peakamplitude in the second signal. In other words, time t₁ is the time fromthe QRS complex, or a component thereof (e.g., Q wave, R wave or S wave)to the peak of the second signal (e.g., the peak of the PPG signal). Anexemplary time t₁ is shown in FIG. 1.

Ventricular depolarization occurs at the beginning of systole, whichsubstantially coincides with the end of diastole (ED in FIG. 1). Themaximum peak amplitude of the second signal occurs when a mechanicalpulse resulting from the ventricular depolarization is detected by thesecond sensor, which is a distance from a location in the patient'sheart where the pulse originated. For example, the second sensor (e.g.,a PPG sensor) can be implanted in the pectoral region, e.g., attacheddirectly to (or by a lead to) the housing of a pacemaker, ICD or cardiacmonitor, as will be described in more detail below. According, the timet₁ is indicative of the time from the beginning of systole (or end ofdiastole) to the peak in the mechanical pulse detected by an implantedsensor.

At step 212, there is a determination of a time t₂ from the detection ofthe ventricular repolarization to the detection of the maximum peakamplitude in the second signal. As just explained, the maximum peakamplitude of the second signal occurs when a mechanical pulse resultingfrom the ventricular depolarization is detected by the second sensor,which is a distance from the location in the patient's heart where thepulse originated. As explained above, the T wave in the first signal(i.e., ECG or IEGM) is indicative of ventricular repolarization.Accordingly, the time t₂ can be determined by determining the time fromthe T wave in the first signal (ECG or IEGM) to the time of the peakamplitude in the second signal (e.g., a PPG signal). Ventricularrepolarization occurs at the end of systole. Accordingly, the time t₂ isindicative of the time from the end of systole to the peak in themechanical pulse detected by an implanted sensor.

At step 214, a peak pulse arrival time (PPAT) is determined based ontimes t₁ and t₂. The diastolic pressure (DP), which is the lowestarterial blood pressure, occurs at the end of diastole (ED in FIG. 1),which substantially coincides with the beginning of systole. Thesystolic pressure (SP), which is the peak arterial blood pressure,occurs during systole, at a time between the beginning of systole andthe end of systole (ES in FIG. 1). In specific embodiments it is assumedthat systole is substantially symmetric, and thus that the peak insystole occurs substantially between the beginning and end of systole.Accordingly, in specific embodiments, PPAT is the mean of times t₁ andt₂. In other words, PPAT can be determined using the equationPPAT=(t₁+t₂)/2. In still other embodiments, slight variations on thisformula can be used. For example, it may be determined that the value 2in the denominator of the PPAT equation should be replaced with 1.8 or2.2, or the like, if it is determined that the peak in systole isslightly asymmetric.

At step 216, a value indicative of systolic pressure (SP) is determinedbased on the PPAT. PPAT is inversely related to SP, in that the greaterthe PPAT the lower the SP, and the lower the PPAT the greater the SP. Ina simplest embodiment, SP≈1/PPAT. However, it would be preferred to usea patient specific correlation factor (e.g., a constant K) whendetermining SP. In other words, in specific embodiments, SP K/PPAT,where K is determined during a calibration procedure. More specifically,an actual value of SP is determined using any known accurate acutetechnique, and a value of PPAT is measured in the manner described aboveusing an implanted system. This will result in K being the only unknownfactor in the equation, and thus, K would be easily calculable (e.g., byan external programmer, or the like). The patient could also be asked toexercise, or could be appropriately paced, to change the patient's SP,to thereby check the accuracy of K over a range of SPs and PPATs. Ifappropriate, K can be adjusted so that K is accurate over a range ofsystolic pressures. Presuming PPAT is measured in msec, the units of Kcan be mmHg·msec, so that when K is multiplied by 1/PPAT, the resultingSP has units of mmHg. Use of look up tables and interpolation are alsowithin the scope of the present invention.

In summary, at step 214, SP can be determined based on PPAT using anequation (e.g., SP=K/PPAT), or using a simple look-up table. Analternative equation could be SP=K/PPAT+β. In a similar manner as justdescribed, β can be determined during a calibration procedure. Otherformulas are also possible, and could be derived by determining actualvalues of the SP for various different values of PPAT, and are withinthe scope of the present invention.

An exemplary calibration procedure (performed at implant and/orthereafter) will now be explained. During the calibration procedure,actual measures of arterial blood pressure, including SP and DP, aremeasured along with values of PPAT (and peak-to-peak amplitude a₁, aswill be discussed below). The actual measure of the patient's SP and DPcan be obtained, e.g., using a non-invasive auscultatory oroscillometric techniques, or an invasive intravascular cannula method,or any other acute technique. For a more specific example, actualarterial pressure measurements (SP and DP) can be measured using a highfidelity micronometer-tipped pressure catheter (e.g., model 4F, SPC-340,available from Millar Instruments, Texas), which is placed in theascending aorta via a carotid arteriotomy. Other techniques are alsopossible, and within the scope of the present invention.

Still referring to FIG. 2A, at step 218, a peak-to-peak amplitude a₁ inthe second signal (e.g., a PPG signal) is determined, at the point ofthe peak amplitude in the second signal (detected at step 208). Forexample, one or more peak detection circuit can be used to detect thepeak-to-peak amplitude a₁. Alternatively, software, hardware and/orfirmware can be used to detect the peak-to-peak amplitude a₁ based onsample data points of the PPG signal, e.g., by determining a differencebetween maximum and minimum sample values of a PPG signal for eachcardiac cycle, or a similar algorithm. An exemplary peak-to-peakamplitude a₁ is shown in FIG. 1. As mentioned above, it would bepractical to perform steps 208 and 218 at generally the same time.

Steps 220 and 222 will now be discussed together. At step 220, a valueindicative of pulse pressure (PP) is determined based on the amplitudea₁. At step 222, a value indicative of diastolic pressure (DP) isdetermined by subtracting the value indicative of PP from the valueindicative of SP (i.e., DP=SP−PP). The value indicative of PP is mainlydetermined so that the value of DP can be determined. Accordingly, steps220 and 222 together can be collectively thought of as determining avalue of DP based on the value of SP (determined at step 216) and thevalue of a₁ (determined at step 218).

Peak-to-peak amplitude a₁ is directly related to the PP, in that thegreater a₁ the greater the PP, and the lower the a₁ the lower the PP. Ina simplest embodiment, PP≈a₁. However, it would be preferred to use apatient specific correlation factor (e.g., a constant M) whendetermining PP. In other words, in specific embodiments, PP=M·a₁, orpossibly PP=M·a₁+σ, where M (and possibly also σ) can be determinedduring a calibration procedure, as will be described below.

During calibration, while actual values of SP are being determined forvarious PPAT values, actual values of DP can also be determined forvarious values of a₁. This will enable the patient specific correlationfactor M (and possibly also a) to be determined during the calibrationprocedure. For example, by combining PP=M·a₁ with DP=SP−PP, a resultingequation is DP=SP−(M·a₁). Since actual values of DP and SP can beobtained during calibration (at implant and/or thereafter), and valuesof a₁ can be measured during calibration, the patient specificcorrelation factor M (and possibly also a) can be easily determined.Other formulas are also possible, and could be derived by determiningactual values of the DP for various different values of a₁. Afterimplant, in similar manners as were discussed above with reference tostep 218, an algorithm or look-up table can be used to calculate PPbased on a₁ at step 220.

Once SP and DP are determined (at steps 216 and 222), mean arterialpressure (MAP) can also be determined. For example, the equation MAP=⅓SP+⅔ DP can be used. Alternatively, the equation MAP=(SP+DP)/2 can beused. Use of other equations is also within the scope of the presentinvention.

In step 214 described above, the peak pulse arrival time (PPAT) isdetermined based on times t₁ and t₂. It is believed that increasedaccuracy can be obtained by using PPAT in step 216 to determine thesystolic pressure (SP), as compared to using a more simple pulse arrivaltime (PAT). Nevertheless, in alternative embodiments, described withreference to FIG. 2B, at step 214′ a pulse arrival time (PAT) isdetermined based on t₁ (but not t₂), and at step 216′ the systolicpressure (SP) is determined based on PAT. In a simplest embodiment,SP≈1/PAT. Alternatively, SP=K/PAT, or SP=K/PAT+β. In a similar manner aswas described above, K (and possibly β) can be determined during acalibration procedure. Other formulas are also possible, and could bederived by determining actual values of the SP for various differentvalues of PAT. Since time t₂ is not used to determine PAT. at step 206′ventricular repolarization need not be detected to determine PAT(however, T waves may be detected for other, unrelated reasons).Additionally, step 212 need not be performed, and is thus not shown inFIG. 2B.

In accordance with specific embodiments of the present invention,arterial blood pressure information such as the value indicative of SP(obtained at step 216), the value indicative of DP (obtained at step222), the value of MAP determined based on SP and DP, and potentiallyother information is stored within memory of the implantable system forlater analysis within the device and/or for later transmission to anexternal device. Such an external device (e.g., an external programmeror external monitor) can then be used to analyze such data.

Embodiments of the present invention are not limited to the exact orderand/or boundaries of the steps shown in FIGS. 2A and 2B. In fact, manyof the steps can be performed in a different order than shown, and manysteps can be combined, or separated into multiple steps. All suchvariations are encompassed by the present invention. For example, steps208 and 218 can be combined into a single step, or step 218 canimmediately follow step 208. For another example, step 206 can beseparated into two steps, one where ventricular depolarization isdetected, and another where ventricular repolarization is detected. Theonly time order is important is where a step acts on the results of aprevious step. For example, PPAT can not be determined at step 216 untiltimes t₁ and t₂ are determined at steps 212 and 214. However, steps 212and 214 can be combined, or their order can be swapped.

In accordance with specific embodiments of the present invention, analarm can be triggered based on comparisons of the values indicative ofSP, the values indicative of DP, the changes in SP and/or the changes inDP to corresponding thresholds. Such an alarm can be part of animplanted system. Alternatively, an implanted system can trigger anon-implanted alarm of a non-implanted system. In still otherembodiments, where arterial pulse pressure information is transmitted(e.g., via telemetry) to an external device, a non-implanted alarm canbe triggered based on comparisons of the values indicative of SP, thevalues indicative of DP, the changes in SP and/or the changes in DP,received by the non-implanted device, to corresponding thresholds.Values indicative of SP and DP can be used to determine values ofindicative of MAP, and corresponding MAP thresholds can be used totrigger alarms or the like.

In accordance with specific embodiments of the present invention, themethod described with reference to FIG. 2A or 2B can be repeated fromtime-to-time, to thereby track changes in SP, DP and/or MAP. Forexample, steps 202-222 can be performed periodically (e.g., once aminute, hour, day, week, or the like). The values indicative of SP, DPand/or MAP can be compared in real time to corresponding thresholds.Alternatively, or additionally, values indicative of SP, DP and/or MAPcan be stored in memory of the implanted system. Such stored values canbe analyzed by the implanted system and/or transmitted (e.g., viatelemetry) to an external system (e.g., external programmer and externalmonitor) and analyzed by the external system. Use of various thresholdscan be used to trigger alarms and/or therapy, as will be describedbelow.

Depending on the frequency, periodic monitoring of arterial bloodpressure may be costly in terms of energy, memory and/or processingresources. Accordingly, it may be more efficient to trigger theperformance of certain steps upon detection of an event, such as aspecific activity, or lack thereof, and/or a specific posture of thepatient. For example, an activity sensor and/or posture sensor can beused to trigger the performance of steps 202-222. For example, steps202-222 can be triggered when it is detected that a patient is inactiveand lying down. Additionally, or alternatively, steps 202-222 can betriggered when a patient is upright and walking. In still otherembodiments, steps 202-222 can be triggered to occur, at specificintervals following a patient changing their posture (e.g., assuming anupright posture) and/or activity level. For example, following atriggering event, values of arterial blood pressure can be determinedonce a minute for 10 minutes, or at 1 minute, 2 minutes, 5 minutes and10 minutes after the triggering event. Of course, other variations arealso possible, and within the scope of the present invention. It mayalso be that one or more specific step, such as step 202, is performedsubstantially continually (e.g., because the signals obtained at step202 are also used for pacing, arrhythmia detection, and the like), butother steps (e.g., steps 204-222) are only performed in response to atriggering event, such as those discussed above.

To detect posture and/or activity, an implantable system can include asensor, which can detect a patient's posture and/or level of activity.The sensor can be, e.g., a DC-coupled 3-dimensional accelerometer asdescribed in U.S. Pat. No. 6,658,292 (Kroll et al), a multi-axis DCaccelerometer as described in U.S. Pat. No. 6,466,821 (Pianca et al), oran external field sensor as described in U.S. Pat. No. 6,625,493 (Krollet al), each of which are incorporated herein by reference. Such sensorsare able to distinguish among different static positions. In addition,since the sensors can detect motion, they can be used to distinguishbetween a static vertical position, such as sitting, and a standingposition, which due to the dynamics of balance is associated with subtlemotion that is not present while sitting. In this way an implantablesystem, using one of the above mentioned sensors or other sensingmodality, can detect a change in body position (i.e., posture), whichcan be used as a trigger to perform specific methods of the presentinvention described below.

It is normal for there to be a normal circadian variation in arterialblood pressure values, including SP, DP and MAP values. For example, adrop in such values when a patient is sleeping, at rest and/or supine isnormal. However, a drop in such values when a patient is active, orupright, or within a short period of a patient assuming an uprightposture, is abnormal. Implanted activity and/or posture sensors can thusbe used to assist in defining when an alarm or the like should betriggered. For example, a posture sensor can be used to trigger themonitoring of arterial blood pressure values when a patient assumes anupright posture. In this manner, such monitoring can be used todetermine whether a drop in blood pressure within a specific amount oftime (e.g., 10 minutes), following the patient assuming of an uprightposition, exceeds a specified threshold. Such a threshold can be, e.g.,an absolute value or a percentage. In specific embodiments, the SP, DPand/or MAP thresholds to which determined SP, DP and/or MAP values arecompared can be based on the activity and/or posture of the patient.

Where at least some of steps 202-222 are triggered in response todetection of various different activity and/or posture states,information about the patient's activity and/or posture can also bestored along with the arterial blood pressure information, so that suchinformation can be correlated. In other words, there could be across-correlation of arterial blood pressure values with levels ofactivity and/or posture.

Additionally, or alternatively, the implantable system can also monitorfor episodes and degrees of myocardial ischemia, and there could be across-correlation of arterial blood pressure values with degrees ofischemia (as well as with levels of activity and/or posture). This canbe useful, e.g., for determining the seriousness associated withischemic episodes. For example, severe ischemia associated with a dropin arterial blood pressure at low levels of activity is more seriousthan a mild degree of ischemia with no drop in blood pressure at highlevels of activity.

In specific embodiments, the implanted system can detect myocardialischemic events based on the ECG/IEGM signals obtained at step 202. Forexample, known techniques can be used that perform ST-segment shiftanalysis to determine if there is a deviation of the ST-segment from abaseline (e.g., a PQ segment baseline), and detect myocardial ischemicevents when the deviation is beyond a threshold. Other techniques arealso possible. The precise technique used to detect episodes ofmyocardial ischemia are not important to the present invention. Rather,what is important is that episodes of myocardial ischemia can bedetected, so that such information can be correlated with arterial bloodpressure information, and preferably information showing suchcorrelations can be stored. For example, the implantable system canstore, in memory, arterial blood pressure data (obtained usingembodiments of the present invention) corresponding to the periodimmediately prior to, during and subsequent to a detected myocardialischemic episode. The implantable device can also store data thatidentifies the ST-segment level during various portions of an episode(e.g., at onset of the ischemia, the peak of the ischemia and thetermination of the ischemia), the time of the ischemic episodes (atonset, at peak and/or at termination), the duration of the episode, aswell as any other type of information that a physician may deem useful,U.S. Pat. Nos. 6,112,116, 6,272,379 and 6,609,023 (all to Fischell etal.), which are incorporated herein by reference, provide exemplaryadditional details of the types of data that can be stored in responseto the detection of a myocardial ischemic episode, and how such data canbe efficiently and effectively stored. Additionally, correspondingarterial blood pressure information, such as values indicative of SP, DPand/or MAP can also be stored. This would enable the implantable system,or an external system and/or physician (which receives the informationfrom the implantable system) to analyze how such conditions areinter-related.

Accordingly, embodiments of the present invention can be used todetermine, or assist with the determination of, whether there is acorrelation between levels of arterial blood pressure, levels ofactivity and/or posture, and myocardial ischemic episodes experienced bya patient. Such information will enable a medical practitioner toanalyze whether ischemic episodes that the patient experienced may haveprecipitated changes in arterial blood pressure, posture and/oractivity.

In accordance with specific embodiments of the present invention,measures of arterial blood pressure, including values indicative of SP,DP and/or MAP, can be used for pacing interval optimization, as well aspacing rate optimization. Exemplary pacing intervals include, but arenot limited to, atrioventricular (RA-RV) delay, interventricular (RV-LV)delay, interatrial (RA-LA) delay and intraventricular (RV1-RV2 orLV1-LV2) delay. This can include adjusting the pacing interval(s) toattempt to maintain the patient's arterial blood pressure at a specifiedlevel(s). The specified level(s) can be an optimal level(s), e.g., asspecified by a physician. In specific embodiments, this can includeincreasing or decreasing specific pacing intervals, or combinationsthereof, to attempt to increase or decrease values indicative ofarterial blood pressure. In other words, measures of arterial bloodpressure, determined in accordance with embodiments of the presentinvention, can be used for closed loop adjustments of pacing parameters.

More generally, measures of arterial blood pressure, obtained inaccordance with embodiments of the present invention can be used toassess the hemodynamic status of a patient. This can include tracking apatient's cardiac disease state, including but not limited to, heartfailure. For example, increases in measures of arterial blood pressureover time can be interpreted as a worsening of a heart failurecondition.

Measures of arterial blood pressure, obtained using embodiments of thepresent invention, can be used for arrhythmia discrimination, includingtachyarrhythmia classification. For example, it is believed that beforethe onset of a tachyarrhythmia, there will be a detectable drop inarterial blood pressure (e.g., DP, SP and/or MAP). If the patient isexperiencing an atrial tachyarrhythmia, it is believed that the arterialblood pressure will return to normal levels as the tachyarrhythmiaprogresses. In contrast, if the patient is experiencing a ventriculartachyarrhythmia, it is believed that the arterial blood pressure willremain low during the ventricular tachyarrhythmia. Accordingly, measuresof arterial blood pressure can be used to distinguish atrialtachyarrhythmias from ventricular tachyarrhythmias. If there areinstances where the arterial blood pressure staying low makes itdifficult to determine PPAT, determinations of PAT may be used in placeof PPAT, in accordance with specific embodiments of the presentinvention.

Measures of arterial blood pressure can also be used to classify atachyarrhythmia as either hemodynamically stable or unstable. Forexample, where arterial blood pressure generally stays within anacceptable range during a tachyarrhythmia, the tachyarrhythmia can beconsidered hemodynamically stable. In contrast, where arterial bloodpressure significantly drops (or increases) due to the tachyarrhythmia,the tachyarrhythmia can be considered hemodynamically unstable. Suchdeterminations of hemodynamic stability can be used to enable, adjustand/or abort certain stimulation therapies, including anti-tachycardiapacing (ATP) and/or shock therapy.

In the past, measures of arterial blood pressure have not generally beenavailable before, at the onset, and during the progression ofspontaneous tachyarrhythmias. By monitoring arterial blood pressure,using embodiments of the present invention, additional information aboutthe relationships between arterial blood pressure and tachyarrhythmiascan be obtained. Such information can be very useful for detecting theonset of tachyarrhythmias, for possibly determining the cause ofspecific tachyarrhythmias, and for selecting, adjusting and/or abortingspecific types of therapy.

FIGS. 3A and 3B will now be used to describe an exemplary implantablesystem that can be used to determine values indicative of arterial bloodpressure, in accordance with embodiments of the present invention.Referring to FIG. 3A, the implantable system is shown as including animplantable stimulation device 310, which can be a pacing device and/oran implantable cardioverter defibrillator. The device 310 is shown asbeing in electrical communication with a patient's heart 312 by way ofthree leads, 320, 324 and 330, which can be suitable for deliveringmulti-chamber stimulation and shock therapy. The leads can also be usedto obtain IEGM signals, for use in embodiments of the present invention.In instead of having leads with electrodes attached to the heart, it isalso possible that subcutaneous electrodes can be used to obtain ECGsignals. In still other embodiments, it's possible that the electrodesare located on the housing of the implantable device 310, and that suchelectrodes are used to obtain subcutaneous ECG signals. In this latterembodiment, the device 310 may not be capable of pacing and/ordefibrillation, but rather, the implantable device 310 can be primarilyfor monitoring purposes.

The implantable system is also shown as including an implantablephotoplethysmography (PPG) sensor 303 that can be used to produce a PPGsignal, similar to signal 108 shown in FIG. 1. Referring to FIG. 3A, thePPG 303 sensor includes a light source 305 and a light detector 307. Thelight source 305 can include, e.g., at least one light-emitting diode(LED), incandescent lamp or laser diode. The light detector 307 caninclude, e.g., at least one photoresistor, photodiode, phototransistor,photodarlington or avalanche photodiode. Light detectors are often alsoreferred to as photodetectors or photocells.

The light source 305 outputs light that is reflected or backscattered bysurrounding patient tissue, and reflected/backscattered light isreceived by the light detector 307. In this manner, changes in reflectedlight intensity are detected by the light detector, which outputs asignal indicative of the changes in detected light. The output of thelight detector can be filtered and amplified. The signal can also beconverted to a digital signal using an analog to digital converter, ifthe PPG signal is to be analyzed in the digital domain. Additionaldetails of exemplary implantable PPG sensors are disclosed in U.S. Pat.Nos. 6,409,675 and 6,491,639, both entitled “Extravascular HemodynamicSensor” (both Turcott), which are incorporated herein by reference.

A PPG sensor can use a single wavelength of light, or a broad spectrumof many wavelengths. In the alternate embodiments, the light source canbe any source of radiant energy, including laserdiode, heated filament,and ultrasound transducer. The detector can be any detector of radiantenergy, including phototransistor, photodetector, ultrasound transducer,piezoelectric material, and thermoelectric material.

It is generally the output of the photodetector that is used to producea PPG signal. However, there exist techniques where the output of thephotodetector is maintained relatively constant by modulating the drivesignal used to drive the light source, in which case the PPG signal isproduced using the drive signal, as explained in U.S. Pat. No.6,731,967, entitled “Methods and Devices for Vascular Plethysmographyvia Modulation of Source Intensity,” (Turcott), which is incorporatedherein by reference.

The PPG sensor 302 can be attached to a housing 340 of an implantabledevice, which as mentioned above can be, e.g., a pacemaker and/or animplantable cardioverter-defibrillator (ICD), or a simple monitoringdevice. Exemplary details of how to attach a sensor module to animplantable cardiac stimulation device are described in U.S. patentapplication Ser. No. 10/913,942, entitled “Autonomous Sensor Modules forPatient Monitoring” (Turcott et al.), filed Aug. 4, 2004 (AttorneyDocket No. A04P3019-US1), which is incorporated herein by reference. Itis also possible that the PPG sensor 302 be integrally part of theimplantable cardiac stimulation device 310. For example, the PPG sensor302 can be located within the housing 340 of an ICD (or pacemaker) thathas a window through which light can be transmitted and detected. In aspecific embodiment, the PPG sensor 302 has a titanium frame with alight transparent quartz window that can be welded into a correspondingslot cut in the housing of the ICD. This will insure that the ICDenclosure with the welded PPG sensor will maintain a hermetic condition.

Where the PPG sensor is incorporated into or attached to a chronicallyimplantable device 310, the light source 305 and the light detector 307can be mounted adjacent to one another on the housing or header of theimplantable device. The light source 305 and the light detector 307 arepreferably placed on the side of the implantable device 310 that,following implantation, faces the chest wall, and are configured suchthat light cannot pass directly from the source to the detector. Theplacement on the side of the device 310 that faces the chest wallmaximizes the signal to noise ratio by directing the signal toward thehighly vascularized musculature, and shielding the source and detectorfrom ambient light that enters the body through the skin. Alternatively,at the risk of increasing susceptibility to ambient light, the lightsource 305 and the light detector 307 can be placed on the face of thedevice 310 that faces the skin of the patient.

The implantable PPG sensor 303 outputs a PPG signal similar to signal108 shown in FIG. 1. More specifically, the output of the light detector305 can be an analog signal that resembles signal 108. Such a signal canbe filtered and/or amplified as appropriate, e.g., to remove respiratoryaffects on the signal, and the like. Additionally, the signal can bedigitized using an analog to digital converter. Based on the PPG signal,and an ECG or IEGM obtained using implanted electrodes, times t₁, t₂ andpeak-to-peak amplitude a₁, which were discussed above with reference toFIGS. 1, 2A and 2B, can be determined, thereby enabling measures of SP,DP and MAP to be determined in accordance with embodiments of thepresent invention.

For much of above description, it has been assumed that theplethysmography sensor used to produce a plethysmography signal is a PPGsensor. Thus, the plethysmography signal has often been referred to as aPPG signal. However, it should be noted that other types ofplethysmography sensors can alternatively be used. Thus, embodiments ofthe present invention should not be limited to use with PPG sensors andPPG signals.

In specific embodiments, the arterial plethysmography signal can beproduced using non-radiant methods and devices, including, but notlimited to mechanical strain, electrical impedance, or pressure. Morespecifically, rather than using a PPG sensor that includes a lightsource and detector, the implanted plethysmography sensor can include astrain gauge, a linear displacement sensor, or an ultrasound transducer,each of which is known in the art. Alternatively, an impedanceplethysmography sensor, which is also known in the art, can be used.Details of exemplary implantable sensors that produce an impedanceplethysmography signals are disclosed, e.g., in U.S. Pat. Nos.4,674,518, 4,686,987 and 5,334,222 (all to Salo), which are incorporatedherein by reference.

Still referring to FIG. 3A, to sense atrial cardiac signals and toprovide right atrial chamber stimulation therapy, the device 310 iscoupled to an implantable right atrial lead 320 having at least anatrial tip electrode 322, which typically is implanted in the patient'sright atrial appendage. To sense left atrial and ventricular cardiacsignals and to provide left-chamber pacing therapy, the device 310 iscoupled to a “coronary sinus” lead 324 designed for placement in the“coronary sinus region” via the coronary sinus for positioning a distalelectrode adjacent to the left ventricle and/or additional electrode(s)adjacent to the left atrium. As used herein, the phrase “coronary sinusregion” refers to the vasculature of the left ventricle, including anyportion of the coronary sinus, great cardiac vein, left marginal vein,left posterior ventricular vein, middle cardiac vein, and/or smallcardiac vein or any other cardiac vein accessible by the coronary sinus.

Accordingly, an exemplary coronary sinus lead 324 is designed to receiveatrial and ventricular cardiac signals and to deliver left ventricularpacing therapy using at least a left ventricular tip electrode 326, leftatrial pacing therapy using at least a left atrial ring electrode 327,and shocking therapy using at least a left atrial coil electrode 328.

The device 310 is also shown in electrical communication with thepatient's heart 312 by way of an implantable right ventricular lead 330having, in this embodiment, a right ventricular tip electrode 332, aright ventricular ring electrode 334, a right ventricular (RV) coilelectrode 336, and an SVC coil electrode 338. Typically, the rightventricular lead 330 is transvenously inserted into the heart 312 so asto place the right ventricular tip electrode 332 in the rightventricular apex so that the RV coil electrode 336 will be positioned inthe right ventricle and the SVC coil electrode 338 will be positioned inthe superior vena cava. Accordingly, the right ventricular lead 330 iscapable of receiving cardiac signals and delivering stimulation in theform of pacing and shock therapy to the right ventricle.

FIG. 3B illustrates an alternative embodiment of the implantable device310. Here a housing 340 of the device is shown as small, thin, andoblong, with smooth surfaces and a physiologic contour which minimizestissue trauma and inflammation. The oblong geometry of the housing 340is desirable because it maximizes separation of electrodes 342 andprevents rotation of the monitor within the tissue pocket, therebyallowing interpretation of morphology features in an ECG sensed usingelectrodes 342. Two ECG electrodes 432 are shown, however more can bepresent. In the alternate embodiment illustrated in FIG. 3C, three ECGelectrodes 342 are present, one at each apex of the triangle formed bythe device housing 340. These three electrodes allow the three standardsurface ECG leads I-III to be approximated. In another embodiment, fouror more ECG electrodes might be used, with each orthogonal electrodepair providing orthogonal ECG signals. Alternatively, an embodimentlacking ECG electrodes is possible. A further alternative has a singleECG electrode with the monitor housing acting as the other electrode inthe pair. U.S. Pat. No. 6,409,675, which was incorporated above byreference, in its discussion of FIG. 2 a-2 c and 3 a-3 c provides someadditional details of an implantable monitor that includes ECGelectrodes on its housing and a PPG sensor. FIGS. 3B and 3C show thatthe implantable device 310 also include a PPG sensor 303. However, theimplantable device 310 can additionally or alternatively include anotherimplantable sensor that obtains an alternative type of plethysmographysignal, examples of which were discussed above.

FIG. 4 will now be used to provide some exemplary details of thecomponents of the implantable devices 310. Referring now to FIG. 4, eachof the above implantable devices 310, and alternative versions thereof,can include a microcontroller 460. As is well known in the art, themicrocontroller 460 typically includes a microprocessor, or equivalentcontrol circuitry, and can further include RAM or ROM memory, logic andtiming circuitry, state machine circuitry, and I/O circuitry. Typically,the microcontroller 460 includes the ability to process or monitor inputsignals (data) as controlled by a program code stored in a designatedblock of memory. The details of the design of the microcontroller 460are not critical to the present invention. Rather, any suitablemicrocontroller 460 can be used to carry out the functions describedherein. The use of microprocessor-based control circuits for performingtiming and data analysis functions are well known in the art. Inspecific embodiments of the present invention, the microcontroller 460performs some or all of the steps associated with determining values ofarterial blood pressure, detecting episodes of myocardial ischemia, andperforming pacing interval optimization. Additionally, themicrocontroller 460 may detect arrhythmias, and select and controldelivery of anti-arrhythmia therapy.

Representative types of control circuitry that may be used with theinvention include the microprocessor-based control system of U.S. Pat.No. 4,940,052 (Mann et. al.) and the state-machines of U.S. Pat. No.4,712,555 (Sholder) and U.S. Pat. No. 4,944,298 (Sholder). For a moredetailed description of the various timing intervals used within thepacing device and their inter-relationship, see U.S. Pat. No. 4,788,980(Mann et. al.). The '052, '555, '298 and '980 patents are incorporatedherein by reference.

Depending on implementation, the device 310 can be capable of treatingboth fast and slow arrhythmias with stimulation therapy, includingpacing, cardioversion and defibrillation stimulation. While a particularmulti-chamber device is shown, this is for illustration purposes only,and one of skill in the art could readily duplicate, eliminate ordisable the appropriate circuitry in any desired combination to providea device capable of treating the appropriate chamber(s) with pacing,cardioversion and defibrillation stimulation. For example, where theimplantable device is a monitor that does not provide any therapy, it isclear that many of the blocks shown may be eliminated.

The housing 340, shown schematically in FIG. 4, is often referred to asthe “can”, “case” or “case electrode” and may be programmably selectedto act as the return electrode for all “unipolar” modes. The housing 340may further be used as a return electrode alone or in combination withone or more of the coil electrodes, 128, 136 and 138, for shockingpurposes. The housing 340 can further include a connector (not shown)having a plurality of terminals, 442, 444, 446, 448, 452, 454, 456, and458 (shown schematically and, for convenience, the names of theelectrodes to which they are connected are shown next to the terminals).As such, to achieve right atrial sensing and pacing, the connectorincludes at least a right atrial tip terminal (A_(R) TIP) 442 adaptedfor connection to the atrial tip electrode 322.

To achieve left atrial and ventricular sensing, pacing and shocking, theconnector includes at least a left ventricular tip terminal (V_(L) TIP)444, a left atrial ring terminal (A_(L) RING) 446, and a left atrialshocking terminal (A_(L) COIL) 148, which are adapted for connection tothe left ventricular ring electrode 326, the left atrial tip electrode327, and the left atrial coil electrode 328, respectively.

To support right ventricle sensing, pacing and shocking, the connectorfurther includes a right ventricular tip terminal (V_(R) TIP) 452, aright ventricular ring terminal (V_(R) RING) 454, a right ventricularshocking terminal (R_(V) COIL) 456, and an SVC shocking terminal (SVCCOIL) 458, which are adapted for connection to the right ventricular tipelectrode 332, right ventricular ring electrode 334, the RV coilelectrode 136, and the SVC coil electrode 138, respectively.

An atrial pulse generator 470 and a ventricular pulse generator 472generate pacing stimulation pulses for delivery by the right atrial lead320, the right ventricular lead 330, and/or the coronary sinus lead 324via an electrode configuration switch 474. It is understood that inorder to provide stimulation therapy in each of the four chambers of theheart, the atrial and ventricular pulse generators, 470 and 472, mayinclude dedicated, independent pulse generators, multiplexed pulsegenerators, or shared pulse generators. The pulse generators, 470 and472, are controlled by the microcontroller 460 via appropriate controlsignals, 476 and 478, respectively, to trigger or inhibit thestimulation pulses.

The microcontroller 460 further includes timing control circuitry 479which is used to control pacing parameters (e.g., the timing ofstimulation pulses) as well as to keep track of the timing of refractoryperiods, noise detection windows, evoked response windows, alertintervals, marker channel timing, etc., which is well known in the art.Examples of pacing parameters include, but are not limited to,atrio-ventricular delay, interventricular delay and interatrial delay.

The switch bank 474 includes a plurality of switches for connecting thedesired electrodes to the appropriate I/O circuits, thereby providingcomplete electrode programmability. Accordingly, the switch 474, inresponse to a control signal 480 from the microcontroller 460,determines the polarity of the stimulation pulses (e.g., unipolar,bipolar, etc.) by selectively closing the appropriate combination ofswitches (not shown) as is known in the art.

Atrial sensing circuits 482 and ventricular sensing circuits 484 mayalso be selectively coupled to the right atrial lead 320, coronary sinuslead 324, and the right ventricular lead 330, through the switch 474 fordetecting the presence of cardiac activity in each of the four chambersof the heart. Accordingly, the atrial (ATR. SENSE) and ventricular (VTR.SENSE) sensing circuits, 482 and 484, may include dedicated senseamplifiers, multiplexed amplifiers, or shared amplifiers. The switch 474determines the “sensing polarity” of the cardiac signal by selectivelyclosing the appropriate switches, as is also known in the art. In thisway, the clinician may program the sensing polarity independent of thestimulation polarity.

Each sensing circuit, 482 and 484, preferably employs one or more lowpower, precision amplifiers with programmable gain and/or automatic gaincontrol, bandpass filtering, and a threshold detection circuit, as knownin the art, to selectively sense the cardiac signal of interest. Theautomatic gain control enables the device 310 to deal effectively withthe difficult problem of sensing the low amplitude signalcharacteristics of atrial or ventricular fibrillation. Such sensingcircuits, 482 and 484, can be used to determine cardiac performancevalues used in the present invention. Alternatively, an automaticsensitivity control circuit may be used to effectively deal with signalsof varying amplitude.

The outputs of the atrial and ventricular sensing circuits, 482 and 484,are connected to the microcontroller 460 which, in turn, are able totrigger or inhibit the atrial and ventricular pulse generators, 470 and472, respectively, in a demand fashion in response to the absence orpresence of cardiac activity, in the appropriate chambers of the heart.The sensing circuits, 482 and 484, in turn, receive control signals oversignal lines, 486 and 488, from the microcontroller 460 for purposes ofmeasuring cardiac performance at appropriate times, and for controllingthe gain, threshold, polarization charge removal circuitry (not shown),and timing of any blocking circuitry (not shown) coupled to the inputsof the sensing circuits, 482 and 486.

For arrhythmia detection, the device 310 includes an arrhythmia detector462 that utilizes the atrial and ventricular sensing circuits, 482 and484, to sense cardiac signals to determine whether a rhythm isphysiologic or pathologic. The timing intervals between sensed events(e.g., P-waves, R-waves, and depolarization signals associated withfibrillation) can be classified by the microcontroller 460 by comparingthem to a predefined rate zone limit (i.e., bradycardia, normal, lowrate VT, high rate VT, and fibrillation rate zones) and various othercharacteristics (e.g., sudden onset, stability, physiologic sensors, andmorphology, etc.) in order to assist with determining the type ofremedial therapy that is needed (e.g., bradycardia pacing,anti-tachycardia pacing, cardioversion shocks or defibrillation shocks,collectively referred to as “tiered therapy”). Additionally, thearrhythmia detector 462 can perform arrhythmia discrimination, e.g.,using measures of arterial blood pressure determined in accordance withembodiments of the present invention. Exemplary details of sucharrhythmia discrimination, including tachyarrhythmia classification, arediscussed above. The arrhythmia detector 462 can be implemented withinthe microcontroller 460, as shown in FIG. 4. Thus, this detector 462 canbe implemented by software, firmware, or combinations thereof. It isalso possible that all, or portions, of the arrhythmia detector 462 canbe implemented using hardware. Further, it is also possible that all, orportions, of the ischemic detector 462 can be implemented separate fromthe microcontroller 460.

Two exemplary types of arrhythmias that the arrhythmia detector 462 candetect include ventricular tachycardia (VT) and ventricular fibrillation(VF). A tachycardia is a fast heart rate (usually over 100 beats perminute) typically caused by disease or injury. It can also be part of anormal response to increased activity or oxygen demands. The averageheart beats between 60 and 100 times per minute. When the tachycardia isdue to disease or injury, it usually requires treatment. Tachycardiasmay begin in the upper chambers of the heart (the atria) or the lowerchambers of the heart (the ventricles). A ventricular tachycardia (VT)begins in the ventricles. Some are harmless, but others are lifethreatening in that they can quickly deteriorate to a ventricularfibrillation.

A ventricular fibrillation (VF) is a very fast, chaotic heart rate(usually over 102 beats per minute) in the lower chambers of the heart,resulting from multiple areas of the ventricles attempting to controlthe heart's rhythm. VF can occur spontaneously (generally caused byheart disease) or when VT has persisted too long. When the ventriclesfibrillate, they do not contract normally, so they cannot effectivelypump blood. The instant VF begins, effective blood pumping stops. VFquickly becomes more erratic, resulting in sudden cardiac arrest. Thisarrhythmia must be corrected immediately via a shock from an externaldefibrillator or an implantable cardioverter defibrillator (ICD). Thedefibrillator stops the chaotic electrical activity and restores normalheart rhythm.

These are just two examples of the types of arrhythmias that thearrhythmia detector 462 can detect. One of ordinary skill in the artwill appreciate that other types of arrhythmias can be detected, andinformation for such other types of arrhythmias can be stored. Examplesof other types of arrhythmias that can be detected by the detector 462include, but are not limited to, supraventricular arrhythmias (SVAs),such as supraventricular tachycardias (SVTs), atrial flutter (AF) and/oratrial fibrillation (AF)

In accordance with embodiments of the present invention, the implantabledevice 310 includes an arterial blood pressure monitor 467, which candetermine values indicative of SP, DP and/or MP, using the techniquesdescribed above with reference to FIGS. 1, 2A and 2B. The arterial bloodpressure monitor 467 can be implemented within the microcontroller 460,as shown in FIG. 4, and can the be implemented by software, firmware, orcombinations thereof. It is also possible that all, or portions, of themonitor 467 to be implemented using hardware. Further, it is alsopossible that all, or portions, of the monitor 467 to be implementedseparate from the microcontroller 460. The arterial blood pressuremonitor 467 can be used in a closed loop control system to provide anassessment of hemodynamic stability during pacing parameter adjustments,and/or as an assessment of hemodynamic stability during a detectedarrhythmia. Such measures of hemodynamic stability can be used whendetermining which anti-arrhythmia therapy options are appropriate.

In accordance with embodiments of the present invention, the implantabledevice 310 also includes an ischemia detector 464, which can detectischemic events based, e.g., on ST-segment shift analysis. The ischemiadetector 464 can be implemented within the microcontroller 460, as shownin FIG. 4. Thus, this detector 464 can be implemented by software,firmware, or combinations thereof. It is also possible that all, orportions, of the ischemia detector 464 can be implemented usinghardware. Further, it is also possible that all, or portions, of theischemia detector 464 can be implemented separate from themicrocontroller 460.

The ischemia detector 464 can monitor sensed cardiac signals in order todetect and record timing and duration information relating to myocardialischemic episodes. Ischemia detector 464 may also trigger a patient orphysician alert in response to detecting a myocardial ischemic event.For example, a patient alert 419, which produces a vibratory or auditoryalert, may be triggered.

There are many documented techniques for detecting episodes ofmyocardial ischemia. Many of these techniques perform ST-segment shiftanalysis to determine if there is a deviation of the ST-segment from abaseline (e.g., a PQ segment baseline), and detect myocardial ischemicevents when the deviation is beyond a threshold. Other techniques arealso possible. The precise technique used by the ischemia detector 464to detect episodes of myocardial ischemia are not important to thepresent invention. Rather, what is important is that the ischemiadetector 464 can detect episodes of myocardial ischemia and causeinformation relating to these episodes to be stored. For example, theimplantable device 310 can store, in memory 494, IEGM data and/orarterial blood pressure data corresponding to the period immediatelyprior to, during and subsequent to a detected myocardial ischemicepisode. The implantable device can also store data that identifies theST-segment level during various portions of an episode (e.g., at onsetof the ischemia, the peak of the ischemia and the termination of theischemia), the time of the ischemic episodes (at onset, at peak and/orat termination), the duration of the episode, as well as any other typeof information that a physician may deem useful. U.S. Pat. Nos.6,112,116, 6,272,379 and 6,609,023 (all to Fischell et al.), which areincorporated herein by reference, provide exemplary additional detailsof the types of data that can be stored in response to the detection ofa myocardial ischemic episode, and how such data can be efficiently andeffectively stored.

The implantable device 310 can also include a pacing controller 466,which can adjust a pacing rate and/or pacing intervals based on measuresof arterial blood pressure, in accordance with embodiments of thepresent invention. The pacing controller 466 can be implemented withinthe microcontroller 460, as shown in FIG. 4. Thus, the pacing controller466 can be implemented by software, firmware, or combinations thereof.It is also possible that all, or portions, of the pacing controller 466can be implemented using hardware. Further, it is also possible thatall, or portions, of the pacing controller 466 can be implementedseparate from the microcontroller 460.

The implantable device can also include a medication pump 403, which candeliver medication to a patient if the patient's arterial blood pressurelevels exceed or fall below specific thresholds. Information regardingimplantable medication pumps may be found in U.S. Pat. No. 4,731,051(Fischell) and in U.S. Pat. No. 4,947,845 (Davis), both of which areincorporated by reference herein.

Still referring to FIG. 4, cardiac signals are also applied to theinputs of an analog-to-digital (A/D) data acquisition system 490. Thedata acquisition system 490 is configured to acquire IEGM and/or ECGsignals, convert the raw analog data into a digital signal, and storethe digital signals for later processing and/or telemetric transmissionto an external device 402. The data acquisition system 490 can becoupled to the right atrial lead 320, the coronary sinus lead 324, andthe right ventricular lead 330 through the switch 474 to sample cardiacsignals across any pair of desired electrodes. In specific embodiments,the data acquisition system 490 may be used to acquire IEGM signals forthe analysis of changes in the ST-segment for detecting myocardialischemia, and for monitoring arterial blood pressure using techniquesdescribed above.

The data acquisition system 490 can be coupled to the microcontroller460, or other detection circuitry, for detecting an evoked response fromthe heart 312 in response to an applied stimulus, thereby aiding in thedetection of “capture”. Capture occurs when an electrical stimulusapplied to the heart is of sufficient energy to depolarize the cardiactissue, thereby causing the heart muscle to contract. Themicrocontroller 460 detects a depolarization signal during a windowfollowing a stimulation pulse, the presence of which indicates thatcapture has occurred. The microcontroller 460 enables capture detectionby triggering the ventricular pulse generator 472 to generate astimulation pulse, starting a capture detection window using the timingcontrol circuitry 479 within the microcontroller 460, and enabling thedata acquisition system 490 via control signal 492 to sample the cardiacsignal that falls in the capture detection window and, based on theamplitude, determines if capture has occurred.

The implementation of capture detection circuitry and algorithms arewell known. See for example, U.S. Pat. No. 4,729,376 (Decote, Jr.); U.S.Pat. No. 4,708,142 (Decote, Jr.); U.S. Pat. No. 4,686,988 (Sholder);U.S. Pat. No. 4,969,467 (Callaghan et. al.); and U.S. Pat. No. 5,350,410(Mann et. al.), which patents are hereby incorporated herein byreference. The type of capture detection system used is not critical tothe present invention.

The microcontroller 460 is further coupled to the memory 494 by asuitable data/address bus 496, wherein the programmable operatingparameters used by the microcontroller 460 are stored and modified, asrequired, in order to customize the operation of the implantable device310 to suit the needs of a particular patient. Such operating parametersdefine, for example, pacing pulse amplitude, pulse duration, electrodepolarity, rate, sensitivity, automatic features, arrhythmia detectioncriteria, and the amplitude, waveshape and vector of each shocking pulseto be delivered to the patient's heart 312 within each respective tierof therapy. The memory 494 can also store arterial blood pressure data.

The operating parameters of the implantable device 310 may benon-invasively programmed into the memory 494 through a telemetrycircuit 401 in telemetric communication with an external device 402,such as a programmer, transtelephonic transceiver, or a diagnosticsystem analyzer. The telemetry circuit 401 can be activated by themicrocontroller 460 by a control signal 406. The telemetry circuit 401advantageously allows intracardiac electrograms and status informationrelating to the operation of the device 310 (as contained in themicrocontroller 460 or memory 494) to be sent to the external device 402through an established communication link 404. The telemetry circuit canalso be use to transmit arterial blood pressure data to the externaldevice 402.

For examples of telemetry devices, see U.S. Pat. No. 4,809,697, entitled“Interactive Programming and Diagnostic System for use with ImplantablePacemaker” (Causey, III et al.); U.S. Pat. No. 4,944,299, entitled “HighSpeed Digital Telemetry System for Implantable Device” (Silvian); andU.S. Pat. No. 6,275,734 entitled “Efficient Generation of SensingSignals in an Implantable Medical Device such as a Pacemaker or ICD”(McClure et al.), which patents are hereby incorporated herein byreference.

The implantable device 310 additionally includes a battery 411 whichprovides operating power to all of the circuits shown in FIG. 4. If theimplantable device 310 also employs shocking therapy, the battery 411should be capable of operating at low current drains for long periods oftime, and then be capable of providing high-current pulses (forcapacitor charging) when the patient requires a shock pulse. The battery411 should also have a predictable discharge characteristic so thatelective replacement time can be detected.

The implantable device 310 can also include a magnet detection circuitry(not shown), coupled to the microcontroller 460. It is the purpose ofthe magnet detection circuitry to detect when a magnet is placed overthe implantable device 310, which magnet may be used by a clinician toperform various test functions of the implantable device 310 and/or tosignal the microcontroller 460 that the external programmer 402 is inplace to receive or transmit data to the microcontroller 460 through thetelemetry circuits 401.

As further shown in FIG. 4, the device 310 is also shown as having animpedance measuring circuit 413 which is enabled by the microcontroller460 via a control signal 414. The known uses for an impedance measuringcircuit 413 include, but are not limited to, lead impedance surveillanceduring the acute and chronic phases for proper lead positioning ordislodgement; detecting operable electrodes and automatically switchingto an operable pair if dislodgement occurs; measuring respiration orminute ventilation; measuring thoracic impedance for determining shockthresholds and heart failure condition; detecting when the device hasbeen implanted; measuring stroke volume; and detecting the opening ofheart valves, etc. The impedance measuring circuit 413 is advantageouslycoupled to the switch 474 so that any desired electrode may be used. Theimpedance measuring circuit 413 is not critical to the present inventionand is shown only for completeness.

In the case where the implantable device 310 is also intended to operateas an implantable cardioverter/defibrillator (ICD) device, it shoulddetect the occurrence of an arrhythmia, and automatically apply anappropriate electrical shock therapy to the heart aimed at terminatingthe detected arrhythmia. To this end, the microcontroller 460 furthercontrols a shocking circuit 416 by way of a control signal 418. Theshocking circuit 416 generates shocking pulses of low (up to 0.5Joules), moderate (0.5-10 Joules), or high energy (11 to 40 Joules), ascontrolled by the microcontroller 460. Such shocking pulses are appliedto the patient's heart 312 through at least two shocking electrodes, andas shown in this embodiment, selected from the left atrial coilelectrode 328, the RV coil electrode 336, and/or the SVC coil electrode338. As noted above, the housing 340 may act as an active electrode incombination with the RV electrode 336, or as part of a split electricalvector using the SVC coil electrode 338 or the left atrial coilelectrode 328 (i.e., using the RV electrode as a common electrode).

The above described implantable device 310 was described as an exemplarypacing device. One or ordinary skill in the art would understand thatembodiments of the present invention can be used with alternative typesof implantable devices. Accordingly, embodiments of the presentinvention should not be limited to use only with the above describeddevice.

The present invention has been described above with the aid offunctional building blocks illustrating the performance of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have often been arbitrarily defined herein for theconvenience of the description. Alternate boundaries can be defined solong as the specified functions and relationships thereof areappropriately performed. Any such alternate boundaries are thus withinthe scope and spirit of the claimed invention. For example, it would bepossible to combine or separate some of the steps shown in FIGS. 2A and2B. Further, it is possible to change the order of some of the stepsshown in FIGS. 2A and 2B, without substantially changing the overallevents and results. For another example, it is possible to change theboundaries of some of the blocks shown in FIG. 4.

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the embodiments ofthe present invention. While the invention has been particularly shownand described with reference to preferred embodiments thereof, it willbe understood by those skilled in the art that various changes in formand details may be made therein without departing from the spirit andscope of the invention.

1. For use with an implantable system, a method for monitoring apatient's arterial blood pressure, the method comprising: (a) usingimplanted electrodes to obtain a first signal indicative of electricalactivity of the patient's heart; (b) using an implanted sensor to obtaina second signal indicative of mechanical activity of the patient'sheart; (c) detecting a ventricular depolarization in a portion of thefirst signal corresponding to a cardiac cycle; (d) detecting a maximumpeak amplitude in a portion of the second signal corresponding to thesame cardiac cycle; (e) determining a pulse arrival time (PAT) bydetermining a time t₁ from detection of the ventricular depolarizationto the detection of the maximum peak amplitude in the second signal; and(f) determining a value indicative of systolic pressure (SP) based onthe PAT.
 2. The method of claim 1, further comprising calibrating theimplantable system by: (i) obtaining accurate measures of the patient'ssystolic (SP) pressure using a non-implanted device and/or an acutelyimplanted device; (ii) using the implanted electrodes and the implantedsensor to determine a pulse arrival time (PAT) corresponding each of aplurality of accurate measures of the patient's SP; and (iii) using theaccurate measures of SP and the corresponding PATs to determine one ormore patient specific correlation factor that enables values indicativeof the patient's SP to be calculated based on PAT; and wherein step (h)includes using the one or more patient specific correlation factor whendetermining the value indicative of SP.
 3. The method of claim 1,further comprising: (g) determining a peak-to-peak amplitude a₁ in thesecond signal; and (h) determining a value indicative of diastolicpressure (DP) based on the amplitude a1 and the value indicative of SP.4. The method of claim 3, wherein step (h) includes: (h.1) determining avalue indicative of pulse pressure (PP) based on the amplitude a₁; and(h.2) determining the value indicative of DP by subtracting the valueindicative of PP from the value indicative of SP.
 5. The method of claim3, further comprising calibrating the implantable system by: (i)obtaining accurate measures of the patient's systolic pressure (SP) anddiastolic pressure (DP) using a non-implanted device and/or an acutelyimplanted device; (ii) using the implanted electrodes and the implantedsensor to determine a pulse arrival time (PAT) and a peak-to-peakamplitude a₁ corresponding each of a plurality of accurate measures ofthe patient's SP and DP; and (iii) using the accurate measures of SP andDP and the corresponding PATs and peak-to-peak amplitudes a₁ todetermine patient specific correlation factors that enable valuesindicative of the patient's SP and values indicative of the patient's DPto be to be calculated based on PAT and a₁, and wherein step (f)includes using at least one of the patient specific correlation factorswhen determining the value indicative of SP; and wherein step (h)includes using at least one of the patient specific correlation factorswhen determining the value indicative of DP.
 6. An implantable systemconfigured to monitor a patient's arterial blood pressure, comprising: afirst detector configured to detect ventricular depolarizations incardiac cycles represented in a first signal that is indicative ofelectrical activity of a patient's heart; a second detector configuredto detect maximum peak amplitudes in cardiac cycles represented in asecond signal indicative of mechanical activity of the patient' heart;an arterial blood pressure monitor configured to determine a time t₁from a detected ventricular depolarization to a detected maximum peakamplitude in the second signal; determine a pulse arrival time (PAT)based on the time t₁; and determine a value indicative of systolicpressure (SP) based on the PAT.
 7. The implantable system of claim 6,wherein: the first signal comprises an intracardiac electrogram (IEGM)or an electrocardiogram (ECG); and the second signal comprises aplethysmography signal.
 8. The implantable system of claim 7, furthercomprising: a sensing circuit to obtain the first signal usingimplantable electrodes; and an implantable plethysmography sensor toobtain the second signal.
 9. The implantable system of claim 8, whereinthe implantable plethysmography sensor comprises a photoplethymographysensor.
 10. The implantable system of claim 6, wherein the arterialblood pressure monitor is also configured to: determine a valueindicative of pulse pressure (PP) based on the amplitude a₁; anddetermine a value indicative of diastolic pressure (DP) by subtractingthe value indicative of PP from the value indicative of SP.
 11. Theimplantable system of claim 10, wherein the arterial blood pressuremonitor can track changes in SP and DP over time.